In recent years, diesel engines have been progressively replacing steam turbines in marine vessels, mainly as a result of the improved economics of the marine diesel. Marine diesel engines may generally be classified as slow-speed, medium-speed or high-speed engines, with the slow-speed variety being used for major, deep draft vessels. Slow-speed diesel engines are typically direct coupled, direct reversing, engines operating in the range of 90 to 250 rpm and usually run on residual fuels. These engines are of crosshead construction with a diaphragm and stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil. Medium-speed engines typically operate in the range of 250 to 1100 rpm and may operate on the four-stroke or two-stroke cycle. These engines are trunk piston design, and many operate on residual fuel as well. They may also operate on distillate fuel containing little or no residua. On deep-sea vessel these engines may be used for propulsion, ancillary applications or both. High-speed diesel engines are comparable to automotive trunk piston diesel engines and are normally employed in deep draft vessels only for special, ancillary applications. These engines generally require high quality distillate fuel oil for satisfactory operation.
In low-speed marine crosshead diesel engines, the cylinders and crankcase are lubricated separately, with cylinder lubrication being provided on a once-through basis by means of injection devices that apply cylinder oil to lubricators positioned around the cylinder liner. The crankcase oil provides lubrication for the bearings, gearing, valve gear and other ancillary engine components and is typically an additive-type oil selected for good oxidation and thermal stability, water demulsability, corrosion protection and good antifoam performance. Alkaline additives may also be present to neutralize any strong acids entering the crankcase through piston rod glands and detergency and extreme pressure (EP) performance may also be provided by the use of suitable additives. Similar performance characteristics are appropriate for the crankcase oils in residual fuel burning, medium-speed trunk piston engines in which the crankcase oil may also be used, in certain types, for splash cylinder lubrication. Other types of medium speed engines may have separate force feed cylinder lubrication. In either case, oil that lubricates the cylinder drains into the sump.
Two specific problems frequently arise in marine diesel engines, namely, contamination of the lubricating oil with blow-by combustion products and with residual fuel components. To a certain extent, blow-by is inevitable since some leakage around the piston rings takes place, especially with larger engine sizes. Crosshead diesel engines, in fact, generally have some blow-by products in the system oil due to the intermingling of the crankcase oil and the cylinder oil in the stuffing box. The commingled oil is generally returned to the crankcase despite the contamination. Lube oil contamination with blow-by products is more direct in medium-speed trunk piston diesel engines, where the system oil and the cylinder oil are the same. Raw residual fuel dilution may occur when seals in engine ancillaries fail to perform adequately, with the result that the chemical composition of the lubricating oil may be altered and sludge formation accelerated with possible fouling of the engine and failure of major components. Normally, a certain degree of fuel dilution is considered acceptable and in most cases up to about 5% dilution can be tolerated. Regardless the type of contamination, frequent monitoring of the engine oil is required to maintain contaminates within acceptable levels.
The formation of sludge is obviously undesirable, and it is very important to determine the cause in order to prevent damage to the engine. This is particularly so with expensive marine engines. Sludge may be formed by fuel contamination of the lubricating oil because the lubricating oil is highly paraffinic whereas the fuel oils have significant aromatic character, which may be relatively immiscible with the lubricating oil. Sludge resulting from raw fuel dilution is probably the polar, highly aromatic fraction of the fuel oil called asphaltenes. This fraction is generally only partly soluble in the lubricating oil. Also because of their aromatic character, asphaltenes have poor combustion characteristics in diesel engines.
Over the past decade, some medium-speed marine diesel engines have experienced problems with excessive deposits, with so-called “black sludge” or “black paint” increasingly found in crankcase and camshaft areas. Likewise, fuel-derived deposits have been found on piston land, ring groove and under-crown areas. Increased availability of residual fuel with declining quality, combined with higher injection pressures in modern marine engines, has led to increased fuel leakage from pumps and injectors into the lube oil sump. Unburned fuel contamination of engine lubricants has become common in these marine diesel engines.
Purifiers are known to assist greatly in cleaning sludge components from the lube oil, but the sludge still must get to the purifier before removal is possible. Unfortunately, once asphaltenes come into contact with metal components, they will often stick to them and remain. In addition, these deposits collect other oil insolubles, such as soot, by providing a sticky surface. Deposits due to asphaltenes may form in the cooler parts of the engine, such as the crank-case or camshaft areas, but they may also form on the hot piston undercrown area resulting in poor cooling of the piston crown.
Deposits and sludge formation, however, are not the only issues of concern in today's medium-speed marine diesel engines. Typically, lube oil viscosity increases and the total base number (TBN) decreases with residual fuel contamination. Oxidation and insolubles generally increase as well. These trends may also be attributed to excessive blow-by of combustion products. Therefore, detection and quantification of residual fuel contamination in used marine diesel engine oils is very important, although it is difficult to determine by conventional used oil analytical tests. Pinpointing the cause of these undesirable lube oil properties is vital for proper assessment of the maintenance action required in a particular case.
Engine builders have been, for several years, redesigning fuel equipment to minimize lube oil contamination. This, combined with the reformulation of trunk piston engine oils (TPEO's) to be more compatible with residual fuel, has resulted in cleaner engines in recent years. Still, high lube oil viscosity and low TBN remain issues today in medium-speed engines, particularly in low lube oil consumption engines. Some engine builders believe that the buildup of pentane (or heptane) insolubles, perhaps more than fuel contamination, is a primary problem. The fact that these insolubles can include asphaltenes, a residual fuel component, tends to inhibit source assessment, however.
Results of a recent study to determine the cause of viscosity increase in modern medium-speed engines indicate that residual fuel contamination is more complex than was earlier thought. Contrary to previous assumptions and observations, fuel contamination is, in at least some modern marine trunk piston diesel engines, more likely the result of unburned or partially burned asphaltenes draining off the liner into the lube oil sump or entering the sump with other blow-by combustion products. The maltenes (non-asphaltenes) fraction of the fuel is completely burned, while at least some of the asphaltenes fraction remains unburned. This study also brought attention to the importance of measuring various types of insolubles in used marine engine oil samples, including those measured by thermogravimetric analysis (TGA). Unfortunately, most methods for measuring insolubles are too time consuming and labor intensive for routine used oil analysis.
U.S. Pat. No. 5,169,785 to Altman et al. discloses a method to detect and quantify unburned residual fuel in marine engine oils based on electron spin resonance (ESR) spectroscopy. Key to the method is the fact that ESR can detect and quantify the vanadium in unburned residual fuel, which is largely present as vanadyl porphyrins. Since the vanadium byproduct from combustion, vanadium pentoxide, is not detected by ESR, any vanadium in used engine oils detected by ESR is from unburned residual fuel. In combination with conventional metals analysis, which measures total vanadium, the ESR method can determine the absolute amounts of vanadium from unburned and combusted fuel. If raw fuel, rather than partially burned fuel, is the dominant contaminant and if the vanadium content of the fuel burned in the engine is known, percent fuel dilution can be calculated. Fuel dilution accuracy is improved when the vanadium content of several recent fuel samples are averaged. This method of analysis is referred to as “Residual Fuel Detection” (RFD). The RFD test has found utility in both technical service applications and in the field-testing of new lubricant formulations. In addition, several engine builders have used it in their own engine testing and component improvement programs. While this test has been well received by industry, it is limited in the information it provides and does not measure fuel contamination directly.
As such, a need exists for a rapid, reliable method for examining residual fuel and insolubles contaminants in marine diesel lubricating oils.